To test the MAFs while driving I measured simultaneous both MAFs. One MAF is connected to the ECU, while both are connected to an analog logging system.

It's difficult to draw a good conclusion as it seems that the airflow is dependent on the position of the MAF sensor. Although of course the total airflow is the same, the uniformity of the airflow isn't. It seems that the mass air flow graph above is accurate, but above 250 kg/hr the curves may cross again.